1. Field of the Invention
The present invention relates to an image coding/decoding method used for coding original images such as a moving image and a still image, and decoding coded images. More particularly, the present invention relates to an image coding/decoding method which can decrease noise for decoding coded images while applying a standardized algorithm such as MPEG-1/2/4 and the like.
In addition, the present invention relates to an image coding apparatus and an image decoding apparatus which perform coding and decoding of images according to the image coding/decoding method.
2. Description of the Related Art
Conventionally, in standard image/moving image coding methods such as JPEG and MPEG series, discrete cosine transform (DCT) has been used for various reasons. Especially, these coding methods has been adopted from the viewpoint of adaptability to image properties.
Normally, when representing an image in the spatial frequency region, there is a property that power concentrates on a low frequency region. The DCT is a method in which a block formed by 8×8 pixels in an image signal (density information) space is orthogonally transformed so that the block is broken down to a combination of predetermined base sets and coefficients of constituent base are obtained. The characteristic of DCT is that degree of bias of coefficients, that is, degree of bias of frequency components can be increased. Especially, compression efficiency can be increased by performing adaptive bit distribution since the degree of concentration is higher in the low frequency region and the low frequency region is visually important.
On the other hand, when coding an image at a very low bit rate, quantization becomes rough so that reproducibility of coefficients becomes worse. Thus, a problem occurs in that the base which is important for representing image signal can not be reconstructed. In addition, DCT performs processing on the region closed in the 8×8 pixel blocks, distortion of coding tends to appear remarkably on boundaries of the block. Due to this tendency, block distortion occurs so that components which are not visually included in the signal appears on the image. Thus, it is detected as noise which is very conspicuous. Especially, in waveform parts where luminance (density) changes abruptly such as in step edges, many bases become necessary for reproducing the image faithfully. However, code assignment for coefficients corresponding to high frequency component is thought to be less important than code assignment for coefficients corresponding to low frequency component in consideration of visual weight. Thus, high frequency coefficients important for reproducing the edge are lost. Due to such processing, conspicuous noise which is intrinsic to DCT appears in the edge part so that image degradation occurs.
In the light of the above-mentioned problem, an image coding apparatus has been proposed in which DCT and differential pulse-code modulation (DPCM) are switched adaptively according to image patterns (for example, Japanese laid-open patent application No.5-276506). This conventional image coding apparatus avoids the signal representation problem of DCT by using DPCM for processing the edge part instead of using DCT.
However, according to the conventional image coding apparatus, the standard coding method using the existing DCT as-is can not be applied. Therefore, some kind of applicable coding method other than the standard coding method is necessary. However, since there are many products which comply with the standard coding method using DCT, it is important to be able to perform coding and decoding of an image by using the standard coding method.